The present invention relates to molded articles having improved flex modulus and impact strength which are produced from a polyurethane material and to a process for the production thereof. The process of the present invention is particularly useful for molding polyurethane elastomers.
In polymeric materials such as polyurethanes, the carbon, nitrogen and oxygen atoms found in the polyurethane linkages are held together by covalent bonds, which result from a sharing of electrons. In this case however, the nuclei do not share the electrons equally, and the electron cloud is denser around certain atoms. The result is that one end of the covalent bond is relatively electron rich (a negative pole) and the other end relatively electron poor (a positive pole). Thus, polyurethane molecules are polar, and the urethane linkage contains a dipole having two equal and opposite charges separated in space. It is therefore theoretically possible that in the presence of a static electric field, the uneven charge distribution in urethane hard segments could make dipole alignment possible.
The general concept of applying an electric field to achieve dipole alignment of selected materials has been disclosed in the prior art. For example, scientists at the University of Chicago and IBM have cooperatively studied the effects of static electric fields on thin films of asymmetric polystyrene-polymethylmethacrylate (PS-PMMA) diblock copolymers. By applying electric fields of approximately 40 kV/cm for 24 hours at elevated temperatures in an inert atmosphere, they were able to achieve alignment of the PMMA microdomain parallel to the electric field. This alignment was locked in by cooling the sample at a rate of 0.5xc2x0 C./min to room temperature and observed using transmission electron microscopy. T. L. Morkved et al, xe2x80x9cLocal Control of Microdomain Orientation in Diblock Copolymer Thin Films with Electric Fieldsxe2x80x9d, Science, Vol. 273, p. 931-933 (Aug. 16, 1996).
Researchers at Cornell University and at Wright Patterson Air Force Base have experimented with alternating electric fields. Using electric field strengths of 10 kV/cm, they observed orientation in cyclic siloxane molecules. Sophisticated synchrotron X-ray equipment enabled them to observe molecular orientation in real time. H. Korner, et al, xe2x80x9cOrientation-On-Demand Thin Films: Curing of Liquid Crystalline Networks in ac Electric Fieldsxe2x80x9d Science, Vol. 272, p. 252-255 (April 12, 1996).
Electric fields are being used in a variety of other applications. Dielectric heating and embossing (U.S. Pat. Nos. 4,482,582; 3,936,412; and 4,423,191), dielectric curing of adhesives (U.S. Pat. No. 5,277,737), monitoring the extent of cure of polymers, filtering to remove contaminants (U.S. Pat. No. 3,324,026), initiation of polymerization reactions (U.S. Pat. No. 3,668,096), and alignment of dipoles in non-linear optical materials (U.S. Pat. No. 5,484,550) are a few examples.
When used to monitor the extent of cure of a polymer, the plate current between two electrodes is measured throughout the reaction. As the material cures, its dielectric loss decreases, thereby decreasing the current between the electrodes.
U.S. Pat. No. 4,482,582 describes a method for altering the dielectric character of polyurethane (xe2x80x9cPUxe2x80x9d) foam to make it more responsive to dielectric heating techniques by pre-treatment with an additive such as polyvinylchloride. U.S. Pat. Nos. 3,936,412 and 4,423,191 disclose post-impregnation with a vinyl plastic or resin or the addition of electrically non-conductive pigments and/or fillers with large dielectric constants to alter the dielectric character of the foam.
It is an object of the present invention to provide a process for producing molded articles having both improved flex modulus and impact strength from polyurethane materials.
It is another object of the present invention to provide molded articles having both improved flex modulus and impact strength.
It is a further object of the present invention to provide molded articles having improved flex modulus and impact strength which need not be treated after demolding.
These and other objects which will be apparent to those skilled in the art are accomplished by subjecting a polyurethane-forming reaction mixture which has been introduced into a mold to a static electric field having a field strength which is less than the dieletric strength of the polyurethane being produced but sufficiently strong to align the hard segment dipoles of the polyurethane.